80% and 90% isn't that much of a difference. Like any other tissue that works harder during increased activity, myocardial oxygen consumption rate will go up during activity because the heart is beating faster and some of that increased oxygen demand could come from taking more O2 out of the blood that is already perfusing the tissue. The key here is that you won't get all that much more oxygen out of that blood even if you increased O2 extraction from 80% to 90% because the basal extraction rate is already very high. Thus, almost all of the increased oxygen supply needs to come from more perfusion with oxygenated blood rather than more efficient extraction of oxygen from existing perfusion.
